This invention relates to an improved rust-preventive coating composition capable of undergoing quick curing by actinic radiation and suitable for use in temporary antirust treatment of steel products, particularly steel tubes and pipes (hereinafter collectively referred to as "steel tubing" or "steel tubes"). It also relates to a steel product having a cured film of such a rust-preventive coating composition on its outer surface.
Various steel tubes including boiler tubes and mechanical tubes as well as other steel products such as sheets, strips, rods, bars, wires, etc. are commonly provided with temporary protective coatings on the outer surfaces thereof throughout the period prior to use in order to eliminate deterioration in quality due to rusting during that period. In the case of tubing, such a temporary protective coating is usually applied in a tube coating line of the tube manufacturing process and removed by the customer prior to use or further processing by appropriate means such as (1) burning-off, (2) physical removal (e.g., shot blasting), or (3) alkaline solution treatment. Rust-preventive coating systems which have heretofore been employed for this purpose include the mineral oil type, the animal or vegetable oil type, and the water-soluble resin type.
These conventional rust preventives involve many disadvantages. For example, mineral oil-type rust preventives provide a coated surface which is permanently sticky because the coatings do not undergo drying or curing. In addition, dripping of the oil from the coated surface may occur during transportation, storage and handling of the coated steel products, resulting in contamination of working environments.
Animal or vegetable oil-type rust preventives such as linseed oil are disadvantageous in that it takes a long time to dry and cure the coatings since the curing generally proceeds via oxidative polymerization in air. Another problem is that this type of rust preventive contains an organic solvent as a viscosity modifier, which evaporates into the surrounding environment to bring about pollution of the environment.
In the case of water-soluble resin-type rust preventives, drying and curing of wet coatings must be effected through time-consuming evaporation of water in a heated zone. As a result, in a high speed coating line of tubes, for example, the coated steel tubes are often passed to a banding and packaging station before the coatings are completely cured. In such cases, the tubes in each package stick to one another in the contact areas and when the customer opens the package and band, the cured films in the contact areas may be peeled off or otherwise damaged. The low drying and curing speed of this type of rust preventive offers another problem in that the rust preventing properties tends to be significantly deteriorated because sagging of uncured coating compositions may readily occur resulting in unevenness of the film thickness, or skid marks may be readily produced while the coated tube is passed over roll skids. As a result, the commercial value of the tubing may be significantly decreased.
In order to overcome the above-mentioned disadvantages of the conventional rust preventives, a method was proposed in Japanese Patent Laid-Open Application No. 142742/1977 wherein a tube is coated on the outer surface thereof with a rust preventive coating composition capable of being cured by actinic radiation such as ultraviolet radiation, an electron beam or the like (such a composition being hereinafter referred to as an "actinic radiation-curable composition") and irradiated with actinic radiation to instantaneously form a cured film. According to such a method using actinic radiation, the rust-preventive coating on the surface of a tube can be completely cured to form a dry film before it contacts the next conveying roll, and therefore the above-mentioned damage to the film caused by contact with rolls or formation of skid marks may be significantly alleviated as compared with the conventional rust preventives. This type of cured film, however, generally cannot be removed by alkali treatment which is a convenient means for removing a temporary coating on steel products.
Japanese Patent Laid-Open Application No. 85233/1979 discloses a method which employs a rust preventive coating composition for metal products comprising 35-95 parts by weight of a mixture of a monofunctional polymerizable compound having one unsaturated group capable of initiating reaction by actinic radiation and a polyfunctional polymerizable compound having two or more such unsaturated groups, with the balance being a copolymer of styrene or its derivative and an unsaturated dicarboxylic acid or its anhydride or ester, preferably a styrene-maleic acid copolymer resin. The coating composition may be applied to a tube and irradiated with actinic radiation to form a cured film. The resulting cured film can be readily removed from the surface of the tube by treatment with a hot alkaline solution prior to use by the customer.
T. Arai et al. in the journal "SUMITOMO METAL", Vol. 35, No. 2, pp. 75-86 (June 1983) discuss various rust-preventive coating compositions for use in temporary protection of steel tubing which are UV radiation-curable and which consist of a base prepolymer, a reactive monomer or oligomer, and a photoinitiator. They particularly discuss the use of a non-reactive film-forming base resin having a carboxylic function such as a styrene-maleic acid copolymer resin of the same type as disclosed in the above Japanese Patent Laid-Open Application No. 85233/1979 in order to render the cured film removable in a hot alkaline solution.
Japanese Patent Laid-Open Application Nos. 110738/1977 and 219272/1984 disclose incorporation of a phosphate ester of a monohydroxyalkyl acrylate or methacrylate in an actinic radiation-curable coating composition in order to enhance the adhesion of the cured film to steel substrate.
Steel tubing frequently bears oil on its surface because various oils or greases are used during processing thereof. For example, in the production of seamless steel tubes, an oil or grease is commonly used in the tube cutting operation to improve efficiency or in the tube straightening step to prevent the straightening rolls from galling. As a result, such oil is inevitably found on the surface of steel tubes and it is usually present as a layer. A naphthenic or paraffinic oil emulsified in water with a surfactant is typically employed for this purpose, although a wide variety of oils are sold on the market.
When one attempts to make a protective coating on a tube which bears oil on its surface by applying thereon an actinic radiation-curable coating composition followed by curing by actinic radiation, it is generally not possible to obtain adequate adhesion or bonding strength between the cured film and the tube for the following reasons:
(1) The setting time of an actinic radiation-curable coating composition is so short that the oil present on the surface of the tube cannot substantially mingle with the coating composition before curing. PA0 (2) Since the cured film is formed almost instantaneously, the stress due to cure shrinkage of the film is also applied instantaneously.
As a result, undesirable phenomena such as "cissing" and "blistering" may be observed on the coated surface.
In order to avoid such adverse effects resulting from the presence of oil on the surface of tubing, in the past it has usually been necessary and the common practice to make the surface as clean as possible prior to coating. Vapor cleaning or alkali degreasing may be employed for this purpose, but incorporation of such a cleaning procedure in the continuous line of tube manufacturing process requires considerable costs for installation and waste water disposal. With a simple degreasing procedure such as physical removal of oil with a wire brush it is difficult to completely remove oil or grease on the steel surface.
T. Arai et al. in the previously mentioned article in "Sumitomo Metal" briefly suggest that a carboxyl-containing non-reactive acrylic resin having a solubility parameter similar to those of mineral oils and a glass transition temperature in a specific range might be employed in place of the styrene-maleic acid copolymer resin as a base resin in order to avoid the loss of adhesion due to the presence of oil on the tube surface. However, the article does not teach any specific numerical limitations on the solubility parameter and glass transition temperature of the resin.
It is well known in the art that it is generally difficult to provide a coating composition having good alkaline removability as well as good adhesion because these properties tend to conflict with each other by nature.
Accordingly, there is still a need for rust-preventive coating compositions suitable for use in temporary protection of steel products which exhibit not only improved adhesion to steel substrates even in the presence of oil but also good alkaline removability